The casing of an in-the-ear hearing instrument is typically constructed from two separate pieces of plastic. The first piece is known as the faceplate 10, depicted in FIG. 1, and the second piece is known as the shell. The faceplate 10 is typically a relatively flat sheet of material and the shell is generally horn-shaped and molded to fit inside a user's ear. The faceplate 10 normally has a variety of features 12 to accommodate the installation of other components. The shell is designed to house the inner workings of the hearing instrument. The inner workings of the hearing instrument are the parts that attach to the features 12 on the faceplate 10.
The faceplate and shell are constructed to mate vertically, so that the shell seats on the faceplate. In addition, the faceplate and shell must be rotationally aligned before being permanently glued together. This alignment often poses an issue for the builder of the hearing instrument.
The shell is glued to the faceplate along the shell wall thickness to join the faceplate to the shell. Once the faceplate is glued to the shell, the faceplate is typically cut to provide a clean connection where the shell is glued to the faceplate. The strength of the shell to faceplate interface is a direct function of the shell thickness. The thicker the shell, the greater the contact area with the faceplate. With soft shells, as the thickness increases, the pliability of the shell decreases. When soft shell assemblies are squeezed, especially near the interface with the faceplate, delamination of the shell from the faceplate is a concern.
One manufacturer, InTech Industries, Inc. of Ramsey, N.J., utilizes a faceplate with small protrusions on the inner surface of the faceplate that assist in aligning the faceplate to a prefabricated hard shell.